The present invention relates to lithium manganese oxides for use in rechargeable lithium and lithium-ion secondary batteries and to methods of making lithium manganese oxides.
Rechargeable lithium and lithium-ion secondary batteries are presently used in portable electronics applications and are potential long-term candidates for powering emission-free vehicles. At present, LiCoO2, LiNiO2 and LiMn2O4 are considered to be the most attractive cathode (positive electrode) materials for use in lithium and lithium-ion batteries. LiNiO2 and LiCoO2 both have high theoretical capacities of about 275 mAh/g. However, the full capacity of these compounds cannot be achieved in practice and only about 140-150 mAh/g can be utilized. Further removal of lithium from LiNiO2 and LiCoO2 further decreases the cycleability of these compounds and causes exothermic decomposition of the oxide. This decomposition releases oxygen at elevated temperatures thus producing safety hazards. LiNiO2, in particular, raises safety concerns because it exhibits a sharper exothermic reaction at a lower temperature than LiCoO2. In addition to these problems, both cobalt and nickel are relatively expensive thus increasing the cost of using these compounds.
LiMn2O4 is often considered a desirable alternative to LiNiO2 and LiCoO2 because it is cheaper and because it is believed to be safer on overcharge. In particular, LiMn2O4 generally has no removable lithium to plate the anode when it is overcharged. Moreover, the end of charge member, MnO2, is believed to be more stable than CoO2 and NiO2, the end of charge members of LiCoO2 and LiNiO2, respectively. Nevertheless, the theoretical capacity of LiMn2O4 is only 148 mAh/g and typically no more than about 115-120 mAh/g can be obtained with good cycleability.
LiMnO2 has traditionally been of great interest for use as a positive electrode material because it has a large theoretical capacity (280 mAh/g). Furthermore, LiMnO2 has a stable end of charge member (MnO2) and is a relatively inexpensive compound to produce. Typically, LiMnO2 is present in an orthorhombic crystalline form. When cycled in a rechargeable lithium or lithium-ion battery, this orthorhombic form converts into a spinel form having a 4V plateau (cubic phase) and a 3V plateau (tetragonal distorted spinel phase). Unfortunately, however, this spinel form loses capacity rapidly when it is cycled through both plateaus. Therefore, this form has not found much utility in lithium and lithium-ion batteries.
In order to provide a LiMnO2 compound that is suitable for lithium and lithium-ion batteries, there have been attempts to produce LiMnO2 with a layered crystalline structure analogous to the layered structure of LiCoO2 and LiNiO2. For example, a LiMnO2 phase has been produced through ion exchange that has the same layered cation distribution as LiCoO2. See A. Armstrong and P. G. Bruce, Nature 381, 499 (1996). The crystalline structure of this layered phase is monoclinic because of the Jahn Teller effect. The stability of the material is poor, however, and the capacity diminishes within only tens of cycles.
Recently, Jang et al. described a layered compound LiMn1xe2x88x92xAlxO2 having a monoclinic crystalline structure and prepared in a solid state reaction at temperatures exceeding 900xc2x0 C. See Y. Jang et al., Electrochemical and Solid-State Letters 1, 13 (1998). The aluminum-doped material produced by Yang et al. was shown to have better cycleability than layered LiMnO2 and was able to sustain more than 20 cycles. But the reversible capacity of this material is only about 110 mAh/g, far below the theoretical capacity. Moreover, after only a few cycles, this material converts to the spinel form having separate plateaus at 3 V and 4 V and thus loses capacity quickly through cycling.
The present invention includes lithium manganese oxide compounds of the formula:
LiMn1xe2x88x92x[A]xO2
wherein 0 less than x less than 0.5, [A] is a combination of two or more dopants, and the average oxidation state N of the dopant combination [A] is +2.8xe2x89xa6Nxe2x89xa6+3.2. For these compounds, N is preferably about +3.0 and 0 less than xxe2x89xa60.4. Preferably, at least one of the dopants is either titanium or zirconium.
In one embodiment of the invention, x=a and [A]x is A1a/2A2a/2. According to this embodiment, A1 is Ti, Zr, or a combination thereof; and A2 is Mg, Ca, Sr, Zn, Ba, or a combination thereof. More preferably, A1 is Ti, A2 is Mg and 0 less than axe2x89xa60.4.
In a second embodiment of the invention, the lithium manganese oxide compound of the invention has the formula LiMn1xe2x88x92bNicA0dA1eA2fO2, wherein A0 is Cr, Co, or a combination thereof; A1 is Ti, Zr, or a combination thereof; A2 is Mg, Ca, Sr, Zn, Ba, or a combination thereof; b=c+d+e+f; 0.1xe2x89xa6bxe2x89xa60.5; 0.1xe2x89xa6cxe2x89xa60.3; 0dxe2x89xa60.4; 0xe2x89xa6exe2x89xa60.2; 0xe2x89xa6fxe2x89xa60.2, and the average oxidation state N of the dopant combination [Ni, A0, A1, A2] is +2.8xe2x89xa6Nxe2x89xa6+3.2. Preferably, in this embodiment, A0 is Cr, A1 is Ti, A2 is Mg, and N is about +3.0.
The present invention also includes lithium and lithium-ion secondary batteries that include the above lithium manganese compounds as the positive electrode material. In addition, the present invention includes dilithiated forms of the lithium manganese oxide compounds of the invention having the formula Li1xe2x88x92zMn1xe2x88x92x[A]xO2, wherein 0xe2x89xa6zxe2x89xa61.
The present invention further includes methods of preparing lithium manganese compounds having the formula LiMn1xe2x88x92x[A]xO2 wherein 0 less than xxe2x89xa60.5, [A] is a combination of two or more dopants, and the average oxidation state N of the dopant combination [A] is +2.8xe2x89xa6Nxe2x89xa6+3.2. The methods of the invention forms these compounds by first mixing together source compounds containing lithium, manganese and [A] in amounts corresponding to the formula LiMn1xe2x88x92x[A]xO2 wherein 0 less than xxe2x89xa60.5, [A] is a combination of two or more dopants, and the average oxidation state N of the dopant combination [A] is +2.8xe2x89xa6Nxe2x89xa6+3.2. The mixture of source compounds is then fired (heated) at a temperature of greater than 700xc2x0 C., and preferably between 800xc2x0 C. and 1000xc2x0 C. to produce the LiMn1xe2x88x92x[A]xO2 compound.